proxy/cluster/Cluster.h is now iocore/cluster/P_ClusterCache.h
Clustering compiles but only works if this module is compiled with the entire
TS because there are a lot of stuff required for clustering in proxy/http2.


cache ifdefs
FIXME_NONMODULAR -> this flaf needs to be turned on when compiling along 
with the entire TS. Turning this flag on should automatically turn the 
HTTP cache flag on.
FIXME_HOSTDB -> Store ifdef to be turned off when HostDB stops using Store
FIXME_MGMT   -> Interface for warnings on the manager
FIXME_CLUSTER_ARGUMENTS -> runtime arguments for clustering etc. Needs
to be added by Eric

cluster ifdefs
FIXME_WIRELESS_API -> RPC API's for wireless which is probably not going to 
be used in the future